Enhanced Stability, Superior Anti-Corrosive, and Tribological Performance of Al2O3 Water-based Nanofluid Lubricants with Tannic Acid and Carboxymethyl Cellulose over SDBS as Surfactant

In this research work, the stability, tribological, and corrosion properties of a water-based Al2O3 nanofluid (0.5 wt%) formulated with tannin acid (TA) and carboxymethyl cellulose (CMC) as dispersants or surfactants were investigated. For comparative purposes, sodium dodecylbenzene sulfonate (SDBS) was also incorporated. The stability of the nanofluid was assessed through zeta potential measurements and photo-capturing, revealing the effectiveness of TA and CMC in preventing nanoparticle agglomeration. Tribological properties were examined using a pin-on-disk apparatus, highlighting the tribofilm of Al2O3 that enhanced lubricating properties of the nanofluid by the SEM, resulting in reduced friction and wear of the contacting surfaces. Sample with the addition of both TA and CMC exhibited the best tribological performance, with a ~ 20% reduction in the friction coefficient and a 59% improvement in wear rate compared to neat nanofluid without TA and CMC. Additionally, the corrosion resistance of the nanofluids were evaluated via weight loss and electrochemical impedance spectroscopy. The nanofluid sample containing both TA and CMC exhibited the lowest corrosion rate, with 97.6% improvement compared to sample without them. This study provides valuable insights into the potential applications of TA and CMC-based Al2O3 nanofluids as effective and environmentally friendly solutions for coolant or lubrication in cutting processes.


Materials
Spherical-shaped Al 2 O 3 with a particle size of 300 nm and purity of 99% was purchased from Ultimate Materials Technology Co., Ltd (Hsinchu, Taiwan).CMC (pH 6.0-8.0) was purchased from Showa Chemical Co., Ltd (Tokyo, Japan).TA was purchased from Daejung Co. (South Korea).Sodium dodecylbenzenesulfonate (SDBS) was purchased from AK Scientific (Union City, USA).

Nanofluid preparation
Table 1 shows the composition of each nanofluid sample.To prepare the AO/SDBS sample, deionized water and Al 2 O 3 (0.5 wt%) were stirred for 30 min, followed by the addition of SDBS (0.15 wt%).Figure 1 shows the preparation procedure for the AO/TA/CMC nanofluid.The AO/TA/CMC nanofluid preparation involved several sequential steps.Firstly, deionized water was combined with 0.15 wt% of TA, stirred at 80 °C for 30 min, ensuring complete dissolution.Subsequently, Al 2 O 3 nanoparticles (0.5 wt%) were gradually introduced into the TA solution, followed by the addition of SDBS (0.15 wt%).Concurrently, a separate solution of CMC (0.15 wt%) was prepared by dissolving it in deionized water.The final step involved slowly combining the CMC solution with the mixture of water, Al 2 O 3 , and tannic acid, and finally, sonicated it with a sonicator for 15 min (600 W).The temperature of the sample was carefully monitored so that it did not exceed 60 °C.The concentrations of all components were determined based on optimal values reported in the existing literature 11,21,23 .

Nanofluid characterization
The particle size of the Al 2 O 3 nanoparticles was characterized using scanning electron microscopy (SEM) (SU-5000, Hitachi, Japan).Approximately 0.1 wt% of the Al 2 O 3 nanofluid was poured onto the surface of a disk.This disk was then placed in a drying oven for several days until all the liquid had evaporated.Then, the dried surface was examined using SEM to accurately determine the particle size of the Al 2 O 3 nanoparticles.To confirm the chemical composition of each nanofluid sample, the Fourier transform infrared spectroscopy (FTIR)(Nicolet iS5, ThermoFisher Scientific, USA) test was performed.

Nanofluid stability (dynamic light scattering and photo capturing)
The stability of each nanofluid was assessed both quantitatively and qualitatively.Dynamic light scattering (Delsa Nano C, Beckman Coulter, USA) was employed to investigate the zeta potential of the nanofluids.Samples were photographed immediately after preparation, after 24 h, after 2, 4, 8, and 15 days.

Tribological performance test
The tribological performance was investigated using a ball-on-disk tribometer (POD-FM406-10NT, Fu Li Fong Precision Machine, Kaohsiung, Taiwan) with a load of 16 N, disk speed of 0.1 m/s, rotation speed of 191 rpm, and sliding distance of 90 m.The disk, with a diameter of 24 mm and a thickness of 10 mm, was made of a Cu-Zn alloy (60% Cu-40% Zn), while the ball material was chrome steel.The hardness and surface roughness of the disk were 120 HV and 3.7 μm, respectively.The Cu-Zn alloy was specifically chosen for the disk to facilitate the detection of Al 2 O 3 particles within the wear scar.The test was carried out in accordance to ASTM G-99 24 .Following the test, the area of the disk scar (depth x length) was examined using a 3D laser scanning microscope.SEM (SU-5000, Hitachi, Japan) with EDS (EDX, JSM-7800 F, JEOL) was employed to investigate and analyze the behavior of the nanofluid inside the scar of the disk.

Anti-corrosion performance test
The corrosion performance of the Cu-Zn disk was investigated using electrochemical impedance spectroscopy (EIS, Jiehan, HIOKI 3533-05) and the weight loss method.The Cu-Zn alloy demonstrates a robust reaction with TA, thereby allowing for a detailed analysis of the corrosion inhibiting effects of this additives 25 .For the EIS method, a liquid composed of 96.5 wt% water and 3.5 wt% NaCl served as the electrolyte.The test was carried out over a frequency range from 100 kHz to 10 mHz, with 5 points recorded per decade.Before conducting www.nature.com/scientificreports/ the test, the Cu-Zn disk was coated with nanofluid by dipping it onto the surface of the disk and then left for 3 days.Afterwards, the particles in the nanofluid will adhere to the surface of the disk and will form a film.The equivalent circuit model for EIS is consistent with previous studies that analyze protective films or coatings 26 .The weight loss method was conducted according to the literature 27 .The disk was immersed in a nanofluid with an addition of 3.5 wt% NaCl to speed up the corrosion process.It was immersed at a temperature of 50 °C for 3 days.The weights of the disk before and after immersion were recorded to determine the weight loss.The weight loss corrosion rate was calculated using the following Equation 16: where v is the weight loss corrosion rate (g/(m 2 h), W a is the weight of specimens before immersion (g), W b is the weight of specimens after immersion without corrosion products (g), S is the surface area of the disk (m 2 ), and t is the total immersion time (h).

Morphology of Al 2 O 3 nanoparticle
The SEM images of the Al 2 O 3 nanoparticles are shown in Fig. 2. It can be seen that the particle exhibits a welldefined spherical shape, as evidenced by the SEM images obtained at a magnification sufficient to discern individual particles.The average diameter of these nanoparticles is measured to be approximately 300 nm, indicating a high degree of uniformity in size across the sample.The choice of a 300 nm size for Al 2 O 3 is aligning with several tribological considerations that enhance their effectiveness on metal surfaces.While smaller nanoparticles (below 100 nm) offer certain advantages like higher surface area and more stable if dispersed in fluid, their effectiveness as lubricant additives might be different 28 .Smaller nanoparticles may lack the load-bearing capacity needed for effective protection under high-stress conditions.Their diminutive size might lead to insufficient film formation, limiting their ability to withstand heavy loads and prevent metal-to-metal contact.This statement is supported by the literature review, which indicates that nanoparticles with a size of 300 nm exhibit a coefficient of friction (CoF) value lower than 0.1, similar to that of smaller nanoparticles 29 .Moreover, smaller nanoparticles are relatively more expensive, as producing small particles is more time-consuming compared to larger ones.

FTIR analysis
Figure 3 displays the FTIR spectra from 4000 to 500 cm −1 for all samples.It can be seen that the main peak for all nanofluids were observed at 3300 and 1600 cm −1 .These peaks are corresponding to stretching vibration of the O-H (hydroxyl) and C=C bond, accordingly 30,31 .After the addition of TA or CMC, the intensity of the 3300 and 1600 cm −1 peak was notably reduced.At the 3300 cm −1 peak, the intensity for the AO/TA/CMC sample decreased by approximately 20% compared to the AO/SDBS sample.This indicates that the addition of TA or CMC, or both, led to lower intensities at these wavelengths.Both CMC and TA has hydroxyl groups (OH), which can potentially form hydrogen bonds with water molecules 32 .
The formation of hydrogen bonds can alter the vibrational characteristics of the OH bonds, leading to changes in the observed FTIR spectrum 33 .The OH stretching vibrations may be affected, resulting in a reduction in intensity.Additionally, this could also possibly be due to electrostatic interactions.

Photo capturing method
Figure 4 shows the photo capturing of nanofluid samples, indicating their stability after preparation and after being left for 30 days.On day 1 (Fig. 4b), or after preparation, all samples exhibit good stability visually.However, after being left for a day, sedimentation begins to be observed for the AO and AO/TA samples.It becomes more visible on day 4 (Fig. 4c), with approximately 50% sedimentation.The samples were fully sedimented by day 7 (Fig. 4d).On the other hand, samples with CMC show relatively good stability.Until day 4, the AO/CMC and AO/TA/CMC samples can prevent sedimentation more effectively compared to those without CMC.The samples maintain stability at around 80% until day 7.After that, the sample started to settle, and they were almost fully sedimented by day 11 (Fig. 4e).On the other hand, in Fig. 4f, the sample  www.nature.com/scientificreports/with CMC needs 30 days for nanoparticles to totally sediment, indicating its stability is far better than the one with SDBS or TA.

Zeta potential method
Table 2 shows the zeta potential value of every nanofluid.According to the literature, all samples has good stability, which indicates with zeta potential value greater than |40| 34 .The negative values indicate that all the particles are negatively charged in suspension.The AO/SDBS sample has a zeta potential value of − 52.85 mV.This value is in accordance with the literature 35 .The AO/CMC and AO/TA/CMC have excellent stability with zeta potential value of − 72.29 and − 60.53 mV, respectively.Compared to AO/SDBS, the AO/CMC zeta potential value was improved by 37%.This significant improvement is attributed to CMC acting as a stabilizing agent in the nanofluid, providing stronger electrostatic epulsion between particles and potentially higher stability in suspension.CMC provides stabilization to the nanoparticles through the charges present in its molecular structure, leading to electrostatic repulsion 20 .These charges impart a negative charge to the Al 2 O 3 , preventing the particles from approaching each other and thereby enhancing stability 36 .The AO/TA sample also has high zeta potential value, which is close to AO/SDBS sample.This may prove that the TA provides steric repulsion to Al 2 O 3 that similar to SDBS .This finding aligns to the previous work research related to using TA for nanofluid stabilization 14 .
However, the AO/TA/CMC did not result in better stability than AO/CMC.This observation may be attributed to the hypothesis that TA might not function optimally in the high electrostatic or charge stabilization region 37 .This study was conducted at a constant room temperature.However, in practical applications, nanofluids are often subjected to fluctuating temperatures that can affect their stability.Temperature variations can alter the solubility of the modifying agents, the viscosity of the fluid, and the kinetic energy of the nanoparticles, potentially leading to agglomeration or changes in zeta potential 28 .Moreover, this study does not investigate the effects of pH and shear rate on the nanofluid system.Both pH and shear rate are critical in real-world applications; pH can influence the surface charge and hence the zeta potential of nanoparticles, while shear rate can affect particle distribution and alignment due to fluid dynamics 28 .Therefore, future research should extend the scope to include these variables to fully characterize the nanofluids under a broader range of conditions.www.nature.com/scientificreports/sliding friction into rolling friction 6 .The AO/TA sample has an average CoF of 0.071, which is slightly lower than AO/SDBS.In the previous work, Uncaria gambir extract (UGE), which has about 50% tannic, was found to provide a more stable CoF profile compared to those without UGE 18,39,40 .Another recent work also reported that adding TA improved or reduced the CoF 41,42 .On the other hand, AO/CMC exhibited an average CoF of 0.064.This is due to the CMC properties that can replenish the base fluid's degraded viscosity and form thicker films between sliding surface, resulting in lower friction 17 .

Tribological properties analysis
Notably, the AO/CMC CoF fluctuates more than others at the first 20 m of sliding distance, indicating a stick-slip effect 43 .Incorporating both TA and CMC results in the lowest CoF, which is 19.7% lower compared to the AO/SDBS sample.This may be attributed to the phenolic compounds of TA that could react with the surface of the disk, forming a protective layer that increases the film thickness.On the other hand, the CMC provides electrostatic stabilization mechanisms that effectively prevent nanoparticle aggregation by reducing the van der Waals force force, ensuring an even distribution and reducing CoF 20,37,44 .
Wear Figure 6 displays the disk's wear rate and wear scar width after undergoing a ball-on-disk tribological test.The surface topography and cross-section depth profile of the wear scar on the disk lubricated with the nanofluid samples are presented in Fig. 7.The wear rate of the AO/SDBS was improved by 5 times compared to water.Adding TA and CMC decreased the wear rate by 19% and 32%, respectively.The disk lubricated with AO/TA/ CMC nanofluid exhibited the lowest wear rate, which was 59% lower than that of the disk lubricated with the AO/SDBS sample.These results are consistent with the CoF values, which show an improvement trend following the addition of additives.
On the other hand, the wear scar width of the rubbed disk also decreased with the increase in sample stability.As seen in Fig. 7, the disk lubricated with neat water has a maximum wear depth of about 18 μm, which is far deeper compared to that lubricated with nanofluid.The AO/SDBS sample exhibits a low depth with a maximum depth of about 2.6 μm.However, it can be seen that the wear scar width is higher compared to disk lubricated with other nanofluids, which explains the high wear rate compared to the disk lubricated with others.This is probably attributable to poorly stabilized AO/SDBS sample that lead to the agglomeration of nanoparticles.When applied as a lubricant, these agglomerated nanoparticles distribute unevenly, causing enhanced surface roughness and resulting in increased friction.
After incorporating TA and CMC, the wear rate and wear scar width are lower compared to the disk lubricated with AO/SDBS.It also can be seen that it has less furrow or relatively low roughness.The reasons for this phenomenon will be further discussed in the following section.Interestingly, although the wear scar width becomes smaller after adding TA and CMC, the wear depth is much deeper than that lubricated with other nanofluids.It can be inferred that the presence of nanoparticles can prevent the direct contact of friction surfaces but may also behave as wear particles, that aggravate abrasive wear at the same time.This phenomenon is similar to one literature that also works using nanofluid 38,45 .
To gain a deeper understanding of TA and CMC behavior in tribological properties, the worn disk surfaces lubricated with AO/TA, AO/CMC, and AO/TA/CMC were investigated using scanning electron microscopy (SEM), as illustrated in Fig. 8.The SEM image of the disk lubricated with AO/TA (Fig. 8a) reveals the presence of cracks on the worn surface, likely due to lower stability compared to the other nanofluids.This uneven stability may result in an inconsistent distribution of aluminum, contributing to localized wear.This observation is supported by the EDS analysis, confirming that only 5.6% of Al is detected.
For the AO/CMC (Fig. 8b), interestingly, the disks has a notably smooth surface.Notably, the presence of a visible tribofilm and Al 2 O 3 particles indicates a mending effect, where the nanofluid contributes to surface repair during tribological contact 46 .The observed smooth surface consistent with the excellent stability of the AO/   opposed to the capacitance in the traditional equivalent circuit model as there was an uneven current potential distribution; CPE would be more accurate according to previous studies 48 .
The EIS fitting results are displayed in Table 3.It is proven that the presence of different additives in the nanofluid affects both R s and R_film.R s values are relatively consistent across all samples.However, there is notable variation in the R_film values, with the AO/TA/CMC combination exhibiting the highest resistance at 99.2 Ω, which was 97.6% higher compared to AO/SDBS.This implies that the synergistic effect of TA and CME in the nanofluid creates a more effective barrier against corrosion compared to others.
The CPE_film-P values are close to unity across all samples, suggesting capacitive behavior with minor deviations.The CPE_film-T values, which are indicative of the time constant of the film capacitance, are lowest for the AO/TA/CME combination, aligning with the highest R_film observed, further confirming the efficacy of this particular nanofluid combination in enhancing the protective film's properties.
Table 4 exhibited the weight loss of Cu-Zn alloy disk immersed in the nanofluid combined with NaCl solution.Comparing these results with the EIS data from Table 3, a clear correlation emerges.Samples with higher R_film values, indicating greater resistance to charge transfer and thus better protection, correspond to lower average weight loss rates.Specifically, the AO/TA/CME sample, which has the highest R_film, also shows the lowest weight loss rate of 0.102 g/(m 2 h). Figure 9b shows the surface of the disk after immersion for 3 days.Lots of corrosion pits were detected on the disk surface after it immersed in AO/SDBS and salt water, indicating that high corrosive reactions occurred.When immersed in TA, only slight corrosion pits and the surfaces were relatively smooth.Immersing with AO/TA/CMC provides smoothest disk surface.This correlation confirms the consistency and validity of the corrosion measurements obtained by both EIS and weight loss methods.
This outcome is a synergistic effect of the combined presence of TA and CMC, that provide enhanced corrosion protection.TA, known for its metal-chelating and antioxidant properties, likely forms a protective layer on the alloy surface, which contributes to its corrosion inhibition potential 49,50 .The incorporation of CMC not only acts as stabilization but also introduces a corrosion protection capability.This dispersion not only enhances the physical barrier formed by TA but also imparts a negative charge to the alloy surface, creating electrostatic repulsion between particles.Incorporating TA and CMC results in a more effective corrosion inhibition.
The differences in impedance between the samples can also be influenced by the dispersion stability of the nanofluids, as reflected by their zeta potential values.The increased stability, as suggested by the more negative zeta potential in the AO/CMC and AO/TA/CMC sample, leads to a lower tendency for particle aggregation, which could contribute to an increase in charge transfer resistance.CMC imparts electrostatic repulsion to Al 2 O 3 nanoparticles.This electrostatic interaction ensures dispersion stability, preventing agglomeration and maintaining an even distribution of nanoparticles across the lubricated surfaces, resulting in well-ordered protective film 53 .Moreover, CMC also acts as a thickener and can enhance the viscosity of the base fluid.This effect offers a dual advantage since increased viscosity is associated with improved stability and tribological properties 53,54 .On the other hand, albeit cannot support CMC as stabilizer, TA is introduced primarily for corrosion protection.Its polyphenolic nature forms a protective layer on metal surfaces, mitigating corrosive reactions.This protective layer acts as a barrier, preventing the degradation of metal components exposed to corrosive environments.
The combined action of CMC and TA yields a synergetic effect on friction reduction.CMC, acting as a thickener, contributes to viscosity control, reducing friction in high-shear conditions 55 .The hydroxyl groups in TA and the oxygen-containing groups in CMC can participate in hydrogen bonding interactions 51 .This bonding may help enhance the adhesion and cohesion of the protective layer, making it more resistant to environmental factors and improving its ability to provide corrosion protection.

Conclusions
In conclusion, we have successfully prepared and demonstrated the effectiveness of TA and CMC in enhancing Al 2 O 3 nanofluid stability and tribocorrosion properties.When compared to the sample without TA or CMC, the lubricated disk exhibits lower coefficients of friction (CoF), lower wear rates, and improved corrosion resistance.The addition of both TA and CMC provides excellent stability and the best CoF improvements, with a 59% reduction compared to the sample without TA and CMC.It also shows the highest corrosion resistance value, with a 97.6% higher resistance and 62% lower mass loss compared to AO/SDBS.TA acts as a corrosion inhibitor due to its polyphenolic nature, forming a protective layer on metal surfaces.On the other hand, CMC provides electrostatic repulsion that counters the attractive forces between nanoparticles.These results indicate that TA and CMC show promising potential as water-based nanofluid additives, offering insights for designing high-performance and environmentally friendly water-based lubricants or coolants for machining applications.
The future potential of TA and CMC holds a promising role as effective nanofluid additives for lubricant or coolant applications, primarily for the enhancement of stability and corrosion prevention.This current research highlights the positive impact of combining TA and CMC into Al 2 O 3 nanofluid.This synergistic combination has demonstrated improvements in stability, tribological, and corrosion properties, contributing to prolonged shelf life and sustained performance.www.nature.com/scientificreports/However, to fully unlock the potential of this nanofluid system, further research is needed.Specifically, investigating the optimal concentration levels for Al 2 O 3 , TA, and CMC in the nanofluid is crucial.Understanding the intricate balance among these components will not only maximize stability, tribological properties, and corrosion resistance but also pave the way for enhanced thermal conductivity and other desirable properties.Furthermore, investigating other properties such as pH, viscosity, stability at high temperatures, electrical properties, and optical properties would be very interesting to conduct.Moreover, it is also necessary to do comparative study with other types of nanofluids or additives that have been available recently.This exploration holds the key to refining the nanofluid formulation for diverse applications, ranging from heat transfer fluids to advanced cooling systems, thereby advancing the frontier of nanofluid technology.

Figure 1 .
Figure 1.Preparation procedure of the water-based nanofluid.

Figure 6 .
Figure 6.Wear rate and wear scar width of disk lubricated with nanofluids.

Figure 9 .
Figure 9.The EIS diagram (a), and Figures of Cu-Zn alloy surface after 3 days immersion (b).

Table 3 .
EIS fitting results of Cu-Zn allow disk continuously immersed in nanofluid for 3 days.

Table 4 .
Weight loss results in solutions with various nanofluid samples after 3 days immersion.

mechanism as surfactant, anti-corrosion additives, and friction modifier
52sed on the investigation, AO/TA/CMC is the best sample due to its excellent, stability and tribocorrosion properties.Lubrication and corrosion mechanism of AO/TA/CMC are proposed and illustrated in Fig.1151.Al 2 O 3 nanoparticles contribute solid lubrication properties, forming a barrier that prevent metal-to-metal surfaces contact, reducing friction and wear52.Incorporating both TA and CMC introduces a sophisticated interplay of tribological mechanisms.